Formation of pyromorphite by hydroxyapatite during lead migration in soil with different phosphorus sorption abilities

Abstract

Chemical immobilization using hydroxyapatite is one of the promising approaches to handling soil with high-level lead contamination. To enhance the effectiveness of lead immobilization by hydroxyapatite, it is important to understand how lead reacts with hydroxyapatite and forms pyromorphite, insoluble lead phases. The up-flow suction percolation test was conducted in this study by the combined application of an X-ray diffraction analysis using fine-grained and sandy soils. The aim of the test was to understand how the availability of lead and phosphorus depends on the formation of pyromorphite in water-unsaturated soil. The results revealed that the available phosphorus derived from hydroxyapatite remained in the sandy soil after the percolation test, but it did not remain in the fine-grained soil. The X-ray diffraction analysis was able to quantify the amounts of lead as pyromorphite formed during the water percolation test. The presence of hydroxyapatite suppressed 48% and 76% of the lead leaching from the contaminated fine-grained and sandy soils, respectively. In the fine-grained soil, much of the pyromorphite formed in the soil near the inflow side; however, as the distance from the inflow side increased, the pyromorphite formation decreased, and lead, as water-soluble, remained. In contrast, in the sandy soil, phosphorus added as hydroxyapatite was extracted, as available. Therefore, pyromorphite homogeneously formed regardless of the distance from the inflow side, resulting in no lead as the water-soluble phases. Thus, hydroxyapatite application for lead immobilization is suitable for soil with a low phosphorus sorption ability.

Keywords

• Available phosphorus
• Immobilization
• Lead mobility
• Pot percolation test
• X-ray diffraction analysis